This study is designed to: (1) estimate the within day component of variance in viral RNA levels in HIV infected subjects; (2) determine whether there is a diurnal pattern in viral RNA levels; (3) determine the relationship between potential circadian patterns of viral RNA and circadian patterns of adrenocortical hormone secretion; and (4) determine the effect of day-to-day variation in viral RNA levels on rates of energy expenditure and substrate oxidation at rest.